Voltage reference circuits that are process, supply voltage, and temperature (PVT) independent have numerous applications. Applications for which PVT independent voltage reference circuits are used include for example forward body bias and analog to digital conversion, as well as any circuits which require accurate supply voltages over a wide range of operating and device conditions.
Conventional voltage reference circuits requiring PVT independence have traditionally used diode or bipolar junction transistor (BJT) bandgap reference circuits. Circuits such as these typically require a supply voltage of at least 1.3 volts. As technology improves, and components become smaller, the supply voltage (V.sub.cc) for processors continues to drop. Some current processor are operating with supply voltages of 1.4 volts. This is close to the limit at which diode or BJT bandgap circuits will become ineffective for use as supply voltages due to the silicon bandgap of 1.23 volts.
As processor supply voltages drop, exploration has begun for the use of different technologies to provide lower supply voltages. Metal oxide semiconductor field effect transistors (MOSFETs) in their subthreshold operation have been used to generate bandgap like reference voltages. The use of MOSFETs in such voltage reference circuits lead to non-linear effects that are brought about by several factors. One of the primary contributing factors to non-linearity in MOSFET based voltage reference circuits is the voltage drop across the depletion depth of the MOSFET. In the threshold voltage equation for a MOSFET, this non-linearity manifests as a body effect term, and affects the behavior of the MOSFET in subthreshold operation.
Transistors such as BJTs and MOSFETs have linear and non-linear dependencies that occur based on a number of factors. Those factors include temperature, process, and supply voltage. If the process changes, the output voltage of the circuit and the way the circuit operates will change. Reasons for the change in output voltage include changes due to devices in the circuit, and changes due to temperature. The changes in device behavior are primarily linear in nature. Changes due to temperature typically include linear and non-linear changes.
Other linear and non-linear effects in MOSFET based voltage reference circuits are similar in nature to linear and non-linear effects in BJT based voltage reference circuits. Such effects include linear temperature dependencies and other non-body effect non-linear temperature dependencies. Methods for linear temperature compensation are known. Methods for compensating for non-body effect non-linear temperature dependencies are also known.
Because of the availability of MOSFET devices to operate at voltages less than typical BJT bandgap voltages, and due to the decreasing supply voltages for integrated circuits and especially processors, there is a need in the art for reducing body effect reference voltage variation across temperature in MOSFET reference voltage circuits.